Organic Chemistry: Benzene and Its Derivates

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The organic reactions involving benzene emphasising on the electrophilic substitution on benzene ring, phenol and aniline.

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Organic Chemistry: Benzene and Its Derivates

  1. 1. Organic Chemistry IV Benzene and Its Derivates Indra Yudhipratama
  2. 2. Outline  Aromaticity  Huckel’s rule  The Reactions (Electrophilic Substitution)  Halogenation  Friedel-Craft’s Reaction  Alkylation and acylation  Nitration and sulphonation  Oxidation and reduction of benzene derivates  Disubstitution (Ortho, meta, para directing groups)  Phenol and aniline  The relative acidity of phenol  The relative basicity of aniline  Diazoniums compounds
  3. 3. The Main Features  The bond length is between C – C and C=C (1.38 A)  Due to delocalised electron (resonance structure)
  4. 4. The Main Features  The structure is planar  Each carbon has p orbital that forms π bonding  Maximum bonding benzene should planar
  5. 5. p Cloud Formation in Benzene
  6. 6. Aromaticity (Hückel’s Rules)  Huckel’s rules define the classification of aromatic and non-aromatic molecule.  The criteria of aromatic molecule:  All the atoms are sp2 hybridised and in planar cyclic arrangement. All atoms are sp2 but not a cyclic. Hence, non-aromatic There is non-sp2 atom. Hence, non-aromatic All atoms are sp2 and a cyclic. Hence, could be aromatic
  7. 7. Huckel’s rules  Huckel’s rule  Number of π-electrons is (4n+2),  How to calculate π-electrons?   based on the structure, p-orbitals in sp2 arrangement has 1 electron Has 6 π-electrons (4n+2, n=1) Hence, aromatic Has 4 π-electrons (4n, n=1) Hence, anti-aromatic
  8. 8. Huckel’s Rule (summary) Is the molecule has no non-sp2 atoms? YES NO How many π- electrons in the molecule? 4n+2 Not 4n+2 aromatic Anti- aromatic non-aromatic
  9. 9. Huckel’s rules Porphyrin ring in the haem group
  10. 10. Huckel’s rule  Which molecules are aromatic?  Is this molecule aromatic? 6 π-electrons 2 π-electrons
  11. 11. The reactions  Benzene undergoes electrophilic substitution  Doesn’t undergo electrophilic addition  The consequence of aromatic properties
  12. 12. The reactions - Halogenation  Halogenation. E.g. chlorination  Via:  The presence of Lewis acid (e.g. AlCl3) helps benzene to react with Cl2
  13. 13. The Reactions – Friedel-Crafts Reaction  Friedel-Crafts Reaction (Alkylation)  To substitute with hydrocarbon chain  Via: Electrophilic generation
  14. 14. The reactions Friedel-Crafts Reaction  There is a problem for this reaction when longer alkyl halide is used  Rearrangement of the electrophile (carbocation)  Trying to find the most stable carbocation
  15. 15. The Reactions  Friedel-Crafts Reaction (Acylation)  To substitute with R-CO –  Via:  Electrophilic generation  acylium ion  stabilised by resonance. Both structures are valid.
  16. 16. The reactions  Acylation can be used to get around the ‘messy’ long chain alkylation.
  17. 17. The Reactions  The nitration (concentrated sulphuric acid as catalyst)  Via:
  18. 18. The Reactions  Sulphonation  Via:
  19. 19. The reactions  Sulphonation  Producing strong sulphonic acid
  20. 20. The Reactions  The Oxidation of toluene  Where R is alkyl group  The Reduction of Aniline R OH O 1) KMnO4, OH- , Heat 2) H3O+ NO2 NH2 Fe HCl aniline
  21. 21. The Reactions  Formation of Diazonium salts  Diazonium salts is a good precursor compound for:  Halogenation  formation of phenol  deamination  coupling reaction of arenediazonium salts NH2 N + N Cl - NaNO2, HCl H2O 0 - 15 o C
  22. 22. The Reactions
  23. 23. The Reactions  Coupling reaction of arenediazonium salts  Where Q is activating group ( –OH, –NR3). E.g.: N + N Cl - + Q N N Q N + N Cl - + OH O + N N H H Cl - OH N N N + N Cl - + OH OH N N
  24. 24. Disubstitution of Benzene  The benzene ring can be substituted with another FG more than once.  The second position is determined by the first FG  Three possible positions: ortho (1,2) meta (1,3) para (1,4) CH3 NH2 OH Cl CH3O OHONH2O NH CH3 O R R1 R R1 R R1
  25. 25. Disubstitution of Benzene  The determining factor  The nature of FG  electron withdrawing (EW) or electron donating (ED) group  EW: the FG generally has partial positive charge  It deactivate the benzene ring, so it is less reactive  ED: the FG generally has partial negative charge  It activate the benzene ring, so it is more reactive
  26. 26. Disubstitution of Benzene
  27. 27. Disubstitution of Benzene  E.g. Application for synthesis route
  28. 28. Phenol  The structure  The relative acidity  Acidity  The easiness to release H+ (proton)  The stability of the acid conjugate determine the relative acidity.  The comparison with water and alcohol (e.g. ethanol) OH
  29. 29. Phenol  Let’s put water as the standard and the conjugate.  More stable the conjugate, more acid the substance.  In ethoxide ions the alkyl group push the electrons  increasing the charge  In phenoxide ions, it forms a bigger resonance structure due to unbonding p-orbital OH O H H CH3 OH O - O - H CH3 O -
  30. 30. Phenol  The effect of substituent  The principle: The reduction of the charge  The deactivating benzene substituent will make phenol more acidic  The activating benzene substituent will make phenol less acidic. Phenol 3-methylphenol 3-nitrophenol 3-chlorophenol pKa = 9.89 pKa = 10.01 pKa = 8.28 pKa = 8.80 OH OH CH3 OH Cl OH NO2
  31. 31. Phenol  Predict the pKa of 2,4 dinitrophenol. (a) 10.17 (c) 8.11 (d) 3.96 (b) 9.31
  32. 32. Phenol  Esterification of Phenol  No reaction with carboxylic acid  Only react with acyl chloride or acetic anhydride OH + CH3 OH O H3O+ No reaction
  33. 33. Phenol  Suggest the products from the reactions below
  34. 34. Phenol  How to distinguish with alcohol?  Since the phenol is more acidic than alcohol, so it can reacts with weaker base (e.g. NaHCO3)  Both of them can react with Na
  35. 35. Aniline  The Basicity of amines  Basicity >< Acidity  Basicity  How easy a compound can accept H+  The case: The relative Basicity of ethylamine, amine, and aniline  The easiness of compound to accept H+  The availability of lone pair electrons on N atom CH3 NH2 NH3 NH2 aniline ammoniamethanamine
  36. 36. Aniline  The reactions  Phenylamine cannot react in the similar way like amine.  Phenylamine is not a better nucleophile than amine  the availability of the electrons on N atom to do the reaction HNO3 concd H2SO4 N + O - O Fe/Sn HCl NH2 NaNO2, HCl 0 - 15 o C N + N Cl - This reaction can produce the other amines. Could you draw the other products?
  37. 37. Aniline  Phenylamine could form an amide with acyl chloride.  Important synthetic pathway for aniline-based compound

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